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(also beak), an organ of birds formed by elongated, toothless mandibles covered with a hornlike sheath, or ramphotheca, which grows continuously. The ramphotheca of some birds is seasonally cast off (for example, birds of the family Tetraonidae). In the embryo of birds a sharp bony protuberance, the egg tooth, temporarily appears near the upper portion of the bill. The egg tooth assists in opening the shell during hatching. In many birds the base of the upper portion of the bill is covered with a waxy substance.
The size and shape of the bill varies in different birds. For some birds the bill assures the capture and, at times, dismemberment of prey; for other birds, the ability to peck and dig. The bill is used to carry out other complex functions, such as the cleaning of feathers and the building of nests. This diversity of functions is made possible by the mobility of the upper portion of the bill, which can be moved up and down or bent in the middle.
Bill-like formations are also found in several mammals (of the subclass Prototheria), reptiles (turtles), and cephalopodan mollusks.
F. IA. DZERZHINSKII